Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 13 Hepatic Function
endogenously produced carbon monoxide is derived from
the enzymatic breakdown of heme (Ryter et al., 2006), and
quantitation of respiratory carbon monoxide has been used
as an indirect measurement of bilirubin production ( Landaw
and Winchell, 1970) . A minor amount of carbon monoxide
arises from other metabolic processes including peroxidation
of lipids (Archakov et al., 2002; Vreman et al.,
1998). In primary hepatocyte cultures, degradation of
hepatic heme also has been shown to use pathways that do
not yield carbon monoxide ( Bissell and Guzelian, 1980 ).
Bilirubin is formed by the catalytic reduction of biliverdin
by the cytosolic enzyme biliverdin reductase, which,
like heme oxygenase, requires NADPH ( Tenhunen et al.,
1970b ). In most mammals, hepatic biliverdin reductase
activity is sufficient and normally is not rate limiting in the
synthesis of bilirubin. Biliverdin reductase activity, however,
is almost completely lacking in birds; consequently,
in avian species, biliverdin is the major pigment of bile.
Biliverdin also is the predominant bile pigment of the rabbit,
in which 70% of bile pigment is biliverdin and biliverdin
reductase activity is low ( George et al., 1989 ; Munoz
et al., 1986 ).
Heme oxygenase is most active in the tissues that
remove erythrocytes and degrade heme. The spleen is the
most important in this regard, followed by the liver and the
bone marrow ( Tenhunen et al., 1968, 1969, 1970a ). The
kidney normally plays a minor role in heme degradation
but in hemolytic disorders associated with hemoglobinuria,
the kidney has a quantitatively more important role in heme
degradation. When intravascular hemolysis begins, glomerular
filtration of hemoglobin is initially prevented by the
binding of hemoglobin to the plasma protein haptoglobin.
When the haptoglobin binding capacity is exceeded, however,
glomerular filtration of hemoglobin occurs. Depending
on the amount filtered, some hemoglobin is reabsorbed by
renal tubular epithelium. This induces formation of heme
oxygenase and production of bilirubin by the kidney ( de
Schepper and Van Der Stock, 1972a, 1972b ; Pimstone
et al., 1971 ). Degradation of hemoglobin in this manner
may have a homeostatic function that conserves iron and
diminishes renal injury associated with hemoglobinuria.
2 . Hepatic Excretion of Bilirubin
Bilirubin is not soluble in aqueous solution and is transported
in plasma from sites of synthesis to the liver bound
to albumin. Bilirubin dissociates from albumin ( Listowsky
et al., 1978 ), and hepatocellular uptake is facilitated across
the basolateral (sinusoidal) plasma membrane by the
organic anion transport polypeptide (OATP), a sodiumand
energy-independent process. OATP also has a role in
the hepatic uptake of a wide range of other organic anions
(e.g., certain drugs, other xenobiotics, sulfobromophlalein
[BSP]) and in the sodium-independent hepatic uptake
of bile salts ( Hata et al., 2003 ). The primary mechanism
for the hepatic uptake of conjugated bile salts is sodium
dependent and utilizes the sodium taurocholate cotransporting
polypeptide (NTCP) ( Hata et al., 2003 ; Jacquemin
et al., 1991, 1994 ; Wolkoff et al., 1985 ).
Localization of OATP in the sinusoidal plasma membrane
of the hepatocyte is determined by PDZ1, a 70-kDa
PDZ binding motif that is present in the liver and kidney.
(PDZ is an acronym that combines the first letters of the
three proteins in which its binding domain was first identified;
Hung and Sheng [2002] .) PDZ domains function in
protein targeting and in the assembly of specific proteinprotein
complexes involved in signal transduction or transport.
In PDZK1 knockout mice, the expression of OATP1a1
in the liver is almost normal, but OATP1a1 is present
only in the cytoplasm and is absent from its normal position
in the plasma membrane. BSP clearance is diminished
in PDZK1 knockout mice, suggesting that oligomerization
of OATP1a1 with PDZK1 determines the subcellular localization
and the function of OATP1a1 ( Wang et al., 2005 ).
Upon entry into the hepatocyte, bilirubin binds to ligandin,
a major cytosolic protein that has both transport and
detoxification functions. Ligandin is a glutathione S-transferase
that catalyzes conjugation of reduced glutathione with
a variety of endogenous substrates and xenobiotics including
porphyrins and certain steroid hormones including cortisol,
BSP, and indocyanine green ( Habig et al., 1974 ; Kaplowitz
et al., 1973 ). By binding bilirubin and inhibiting the efflux
of pigment back into the plasma, ligandin serves as a driving
force for initial hepatic uptake ( Listowsky et al., 1978 ).
Conjugation of bilirubin with glucuronic acid is catalyzed
by bilirubin uridine-diphosphate glucuronosyltransferase-1
(BUGT1) in a reaction in which one or both
propionic acid side chains of bilirubin IX are esterified.
BUGT1 is a microsomal isoenzyme that produces watersoluble
bilirubin mono- and diglucuronide. The UGT family
of enzymes is among the most important mammalian
detoxification mechanisms.
Bilirubin diglucuronide represents more than 80% of the
total bile pigment in healthy adult human bile. Glucuronic
acid esters of bilirubin have been identified in the bile of
a variety of species including the dog ( Talafant, 1956 ), rat
( Grodsky and Carbone, 1957 ), guinea pig ( Schmid, 1956 ),
and in the bile of the horse, pig, cat, sheep, and cattle
(Cornelius et al., 1960 ). Gordon et al. (1976) have shown
that the diglucuronide is the major bilirubin conjugate
excreted in canine bile. There is evidence, however, that
the bile of dogs ( Fevery et al., 1971 ; Heirwegh et al., 1975 ;
Noir, 1976 ) and other species ( Cornelius et al., 1975a,
1975b ) also contains bilirubin conjugates of glucose and
xylose.
When the activity of bilirubin-UGT in the liver is diminished,
production of bilirubin mono- and diglucuronide
decreases. Reduction of conjugating enzyme activity to